The present invention relates generally to systems and methods for performing noninvasive surgical procedures using focused ultrasound, and more particularly to systems and methods for testing and calibrating a focused ultrasound transducer array.
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby obviating the need for invasive surgery. For this purpose, piezoelectric transducers driven by electric signals to produce ultrasonic energy have been suggested that may be placed external to the patient but in close proximity to the tissue to be ablated. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused at a xe2x80x9cfocal zonexe2x80x9d corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of sonications is used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., has been suggested for this purpose. This spherical cap transducer array includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or xe2x80x9csectors,xe2x80x9d creating a sector-vortex array. The transducer elements are generally simultaneously driven by radio frequency (RF) electrical signals at a single frequency offset in phase and amplitude. In particular, the phase and amplitude of the respective drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired xe2x80x9cfocal distance,xe2x80x9d i.e., the distance from the transducer to the center of the focal zone, and/or to provide a desired energy level in the target tissue region. In addition, the phase of the respective drive signals to each of the sectors may be controlled to create a desired size and shape for the focal zone.
Transducer arrays are generally composed of numerous transducer elements that may be difficult and/or costly to fabricate and require complex drive circuitry and hardware to control and power. As part of its initial production and assembly, a focused ultrasound system is generally tested and configured, for example, to ensure that the individual transducer elements of the transducer array and/or the drive and control circuitry perform properly.
Once in operation, the system may be susceptible to degradation in performance and/or possible failure of some of the transducer elements. This degradation may be caused by normal aging processes and/or by misuse of the system. For example, the piezoelectric material forming the transducer elements may age, possibly changing their impedance or efficiency. Likewise, problems may develop in the drive circuitry during the life of the system.
Accordingly, it would be desirable to monitor and/or test the performance of a focused ultrasound transducer array to ensure its ongoing proper performance.
In accordance with a first aspect of the present invention, a method is provided for testing the performance of a focused ultrasound transducer array. An acoustic reflector is located at a position to receive ultrasonic energy transmitted by the transducer array. Ultrasonic energy is transmitted from the transducer array, the ultrasonic energy is reflected off of the reflector, and is received by a sensing element. The performance of the transducer array is then evaluated based upon the received reflected ultrasonic energy.
The reflector is preferably provided with well-defined and predictable ultrasonic reflection characteristics, and may be positioned at any location within the acoustic view of the transducer array. In one embodiment, the reflector may be a planar acoustic reflector, such as an xe2x80x9cair mirror,xe2x80x9d placed between the acoustic fluid in which the transducer is disposed and the air above the surface of the acoustic fluid. In an alternative embodiment, the reflector may be a curved reflector or a point reflector. For a concave or xe2x80x9cspherical capxe2x80x9d transducer array, a planar reflector may be located between the transducer and its geometric focal point, preferably half-way between them. In this arrangement, incidental ultrasonic energy may be reflected off of.the reflector to a single point located at the center of the transducer array, i.e., to its xe2x80x9cvirtualxe2x80x9d geometric focal point. Alternatively, if a point reflector is used, it may be located at the actual geometric focal point of the transducer array.
Thus, the reflected ultrasonic energy may be received, for example, at the virtual geometric focal point of the transducer array. The performance of the transducer array may then be quantified based on an analysis of the received ultrasonic energy. Preferably, the performance of the transducer array is quantified by exciting individual transducer elements in the transducer array and comparing one or more actual characteristics of the received ultrasonic signals, e.g., gain and/or delay, to expected characteristics of the received ultrasonic signals. The expected characteristics of the received ultrasonic signal may be obtained from an acoustic model of the testing system. In a preferred method, wherein a planar reflector is located half-way between the transducer array and the geometric focal point, the acoustic wave modeling is facilitated because the bore-sight of the reflected ultrasonic energy is incident at the point of reception, regardless of the location of the individual transducer element from which it originates. Thus, no off-bore-sight sight calculations need be made. The quantified performance of the transducer array may then be used, for example, to calibrate the transducer array and/or to declare a system failure should the performance of the transducer array be severely degraded.
In accordance with a second aspect of the present invention, a method is provided for testing a focused ultrasound transducer array having a plurality of transducer elements. An acoustic reflector, such as a planar reflector, is positioned adjacent the transducer array, and a plurality of reflected ultrasonic signals are produced by transmitting a plurality of ultrasonic signals from the plurality of transducer elements towards the acoustic reflector. The ultrasonic signals may be transmitted by exciting individual transducer elements or a set of transducer elements. As previously described, the transducer array may have a concave or spherical cap shape, and the acoustic reflector may be an xe2x80x9cair mirrorxe2x80x9d located half-way between the transducer array and its geometric focal point.
The plurality of reflected ultrasonic signals may be received and one or more actual ultrasonic reflection characteristics, such as gain and/or delay, may obtained from each of the plurality of received ultrasonic signals. The plurality of actual ultrasonic reflection characteristics may then be compared with a plurality of expected ultrasonic reflection characteristics.
Preferably, the delay of each of the received ultrasonic signals with respect to the transmitted ultrasonic signals is determined, which may be an actual time delay (for pulsed signals) or a phase shift between the signals (for sinusoidal signals). Alternatively or in addition, the gain of each of the received ultrasonic signals may be obtained by comparing the amplitudes of the plurality of received ultrasonic signals with the corresponding amplitudes of the plurality of transmitted ultrasonic signals. This may be accomplished, for example, by comparing electrical signals produced by a sensing element representative of the received ultrasonic signals with electrical signals used to generate the transmitted ultrasonic signals, i.e., the corresponding electrical signals used to excite the transducer elements into producing the plurality of ultrasonic signals.
The actual ultrasonic reflection characteristics may be compared with a respective plurality of expected ultrasonic reflection characteristics. As previously described, the expected characteristics of the received ultrasonic energy may be obtained from an acoustic model of the testing system. The transducer array may then be calibrated, or alternatively, a system failure may be declared, based upon the ultrasonic reflection characteristic comparison.
In accordance with a third aspect of the present invention, an ultrasound system is provided that includes a focused ultrasound transducer for emitting ultrasonic energy, a passive acoustic reflector for reflecting ultrasonic energy emitted by the transducer, and an ultrasound sensing element for sensing the reflected ultrasonic energy. The transducer may, for example, be a phased transducer array used to treat abnormal tissue within a patient. In a preferred embodiment, the shape of the transducer takes the form of a spherical cap, but alternatively may take on any suitable shape that enables the focusing of ultrasonic energy. More preferably, the reflector is an air mirror located half-way between the transducer and its geometric focal point. Thus, in this embodiment, the sensing element may be located at a single point at the center of the transducer. The geometry of the reflector may be any shape that provides well-defined and predictable acoustic reflection characteristics, and may be located anywhere within the view of the transducer.
The system further includes processing circuitry coupled to the sensing element for measuring actual characteristic(s) of the received ultrasonic signals, e.g., the amplitude and/or phase. The processing circuitry may also be coupled to drive circuitry used to excite the transducer into emitting the ultrasonic energy. In this embodiment, the processing circuitry may compare the amplitude and phase of the received ultrasonic signal with the amplitude and phase of the emitted ultrasonic signal.
The system further includes a controller coupled to the processing circuitry for comparing the actual characteristic(s) of the received ultrasonic energy, e.g., the gain and/or delay (or phase shift), to expected characteristic(s) of the sensed ultrasonic energy. The controller and processing circuitry may both be a single integral component, such as a personal computer with special boards added (such as acquisition boards), or other microprocessor, or alternatively, may be separate components. The controller may then use the difference information to calibrate the emitting transducer and/or to declare a system failure if need be.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.